When an oil or gas well is drilled, it is often desired to test the production capabilities of the subsurface formations intersected by the well by lowering a test string into the borehole to the formation depth. The formation fluid is then allowed to flow into the test string in a controlled testing program. It is further known to actuate one or more of the tools in the test string by increasing the well annulus pressure.
One annulus pressure operated testing tool that is commonly included in the test string is a combination safety and circulating valve. When actuated, a safety circulating valve closes in the formation by closing a closure valve, and simultaneously opens the string to fluid flow from the well annulus by opening a circulating port above the closure valve. Various valves of this nature are disclosed in U.S. Pat. Nos. 4,270,610, 4,311,197, 4,444,571, 4,691,779, and 4,657,083. Such devices may include sampling capability, through the use of pairs of spaced ball valves that trap a sample of the flowing fluid therebetween. In these tools, the ball valves themselves can be referred to as closure or safety valves, since they operate to shut off the flow of well fluid through the test string. Safety circulating valves typically include a cylindrical housing within which a concentric mandrel is driven from an first position to a second position upon actuation of the valve.
The valves disclosed in the patents cited above are referred to as atmospheric referenced tools because the differential area piston which drives the mandrel has a low pressure side exposed to substantially atmospheric pressure. In annulus pressure operated tools, the high pressure side of the piston is exposed to annulus pressure during operation. In each of the tools cited above, the low pressure side of the piston comprises a sealed chamber, created when the tool is assembled, which contains air at atmospheric pressure. Although that pressure may change due to heating or cooling after the tool is placed in a well, such changes are negligible for present purposes. An example of the low pressure chamber is shown at reference numeral 80 in FIG. 2B of U.S. Pat. No. 4,270,610.
When in use, the test string is subjected to pressure both inside the test string and in the well annulus. In the absence of additional applied pressure, both regions are subjected to hydrostatic pressure due to the weight of the drilling mud at the string depth. When it is desired to actuate an annulus pressure operated tool such as those described above, an additional pressure on the order of 1,000 to 5,000 psi is applied to the annulus. In an atmospheric referenced tool, the differential pressure that drives the mandrel is equal to the sum of the hydrostatic pressure and this applied annulus pressure. In most cases, the prior art tools will function satisfactorily under these conditions.
In certain high-pressure wells, however, it is also desired to test the integrity of the equipment under high pressure conditions by applying an increased pressure to the inside of the test string before starting the flow portion of the drill stem test. For example, such pressure tests can require applied pressure at the surface on the order of 10,000 to 20,000 psi. Because the total pressure inside the test string at the bottom of the well is equal to the sum of the applied pressure and the hydrostatic head, the pressure differential across the mandrel between the test string interior and the low pressure (atmospheric) chamber outside the mandrel can be as high as 35,000 psi.
While the prior art valves function satisfactorily when used in conventional wells wherein the pressure differential across the mandrel is always less than about 25,000 psi, it has been found that the higher differential that exists during high pressure equipment tests can result in permanent deformation of the mandrel. This is because the annular low pressure air chamber forms a large, unsupported area around the mandrel, into which the increased interior pressure tends to force the mandrel. With nothing to support it, the mandrel bulges radially outward. Deformation to this degree causes the metal of the mandrel to yield plastically, resulting in permanent deformation.
The consequences of this deformation may be severe, as the deformed mandrel cannot function as intended and may be completely immobilized relative to the housing. If the mandrel cannot slide longitudinally as intended, the ability to operate the closure and circulating valves is lost. More importantly, the ability of the tool to operate as a safety valve is compromised. Hence, a safety/circulating valve is desired that is not rendered inoperable by high pressure test conditions.